powerpc/pseries: Disable IBMEBUS on little endian builds

The IBMEBUS code supports the GX bus found on Power7 and earlier CPUs.
On Power8 it has been replaced, and so we have no need for it.

We don't actually have a config symbol for Power8 vs Power7 etc., but
we only support booting little endian on Power8 or later, so use that as
a reasonable approximation.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>